Experimental protocols to identify and quantify nitrogen nutrient deficiency in the activated sludge process were developed and tested using respirometry. Respirometric experiments showed that when a nitrogen nutrient deficient sludge is exposed to ammonia nitrogen, the oxygen uptake rate (OUR) of the sludge increases while an initial nitrogen nutrient uptake takes place at the same time. Our investigation suggests that this initial nitrogen uptake is metabolically mediated. The protocols developed in this study can be used: (a) to assess whether a sludge sample is nitrogen nutrient deficient or not; and (b) to estimate the amount of nitrogen required to remedy a nutrient deficient condition in the sludge. Finally, a respirometry-based strategy to control nutrient addition to the activated sludge process is proposed. (C) 2000 Elsevier Science Ltd. | Experimental protocols to identify and quantify nitrogen nutrient deficiency in the activated sludge process were developed and tested using respirometry. Respirometric experiments showed that when a nitrogen nutrient deficient sludge is exposed to ammonia nitrogen, the oxygen uptake rate (OUR) of the sludge increases while an initial nitrogen nutrient uptake takes place at the same time. Our investigation suggests that this initial nitrogen uptake is metabolically mediated. The protocols developed in this study can be used: (a) to assess whether a sludge sample is nitrogen nutrient deficient or not; and (b) to estimate the amount of nitrogen required to remedy a nutrient deficient condition in the sludge. Finally, a respirometry-based strategy to control nutrient addition to the activated sludge process is proposed.